1. Field of the Invention
The present invention relates to an optical element having a minute periodic structure with a period smaller than the wavelength of incident light.
2. Description of the Related Art
Various optical elements are used in optical systems for use in an optical apparatus such as a silver-haloid camera, a digital still camera, a video camera, and a projector.
When light passes through an interface between media with different refractive indexes, for example between air and glass, Fresnel reflection occurs to cause degraded optical performance.
An approach to suppress the Fresnel reflection is to provide a reflection suppressing function by applying a low refractive index material such as MgF2 and SiO2 with an appropriate thickness to a surface of an optical element. In addition, an approach recently proposed is to use a reflection suppressing function provided by a minute periodic structure with a period smaller than the wavelength of incident light.
Such an element with the reflection suppressing function having a minute periodic structure has been proposed in Japanese Patent Application Laid-Open No. S58 (1983)-174906, Japanese Patent Application Laid-Open No. H1 (1989)-252902, and Japanese Patent Application Laid-Open No. 2001-183506.
In each of such elements with the reflection suppressing function, since a minute periodic structure is formed directly on a substrate, more flexible design is allowed because of no limitations on the low refractive index material as compared with the case where the thin film is used. When molding is employed to simultaneously form the substrate and the minute periodic structure, the advantage of cost is provided.
The aforementioned proposals, however, do not clearly describe the specific uses of the elements with the reflection suppressing function having minute periodic structures.
By way of example, consider a single lens having a thickness of 10 mm, a radius of curvature R1 of a surface on an object side equal to 100 mm, a radius of curvature R2 of a surface on an image plane side equal to −100 mm, a refractive index Nd of a material equal to 1.84666, and an Abbe number vd equal to 23.8.
A characteristic curve A in a graph of FIG. 9 represents spectral transmittance as an optical system when the single lens have neither a minute periodic structure nor an anti-reflection film formed thereon. In the graph of FIG. 9, the horizontal axis represents a wavelength, and the vertical axis represents the spectral transmittance.
If the optical system having the single lens is used to take images with a silver-haloid film, color representations in taken images are determined by the spectral transmittance of the optical system and the sensitivity of the film. In the spectral transmittance shown as the characteristic curve A in FIG. 9, the transmittance is relatively low at shorter wavelengths, so that white color does not appear as pure while in taken images but looks as yellowy white.